The Lamego Cistern, located outside the Plaza de Armas, is made of rectangular brick and vaulted with a pointed arch, supported by torso arches supported by pillars attached to the side walls. About twenty meters long and ten meters wide, it is considered one of the best examples of cisterns in Portuguese castles.

It is a large reservoir with a vaulted structure built in granite ashlars. The pavement of the upper terrace, tiled, has a double slope allowing the capture of rainwater into the cistern. It also has four openings that, over successive generations, allowed the urban population to obtain the water necessary for their daily activities.